Embodiments of the present invention relate to piezoelectric resonator structures and a method for manufacturing coupled resonator devices and in particular to a frequency trimming of a coupled resonator device.
A bulk acoustic wave (BAW) filter, which comprises a stack with a piezoelectric layer separating a top and a bottom electrode, is an example for a simple piezoelectric resonator. In addition to the simple piezoelectric resonator, coupled resonators or coupled resonator filters (CRF) are also known. As a special implementation of BAWs, the CRFs comprise two BAW resonators generally arranged one on top of the other in a stack. In manufacturing of CRFs a narrow specification with respect to the frequency position (resonance frequency) is needed, particularly for their use as bandpass frequency filters for mobile telecommunication applications (mobile phones). Based on the state of the art accuracy of thin film deposition processes, it is currently not possible to produce a CRF with a reasonable manufacturing yield, relying on deposition accuracy alone. This is particularly the case when specifications of the mobile phones are taken into account. For this reason, one or more additional frequency correction steps (trimming) are used to compensate for inaccuracies, e.g., with respect to layer thickness in thin film deposition processes.
Currently, CRFs have not been used in mobile phones yet. Instead, surface acoustic wave filters (SAW) and recently also the BAW filter with a single piezoelectric layer are used. Normally, SAWs do not need any frequency correction, because their frequency position is mainly defined by a pitch of the interdigital structure, which can be controlled with high accuracy by means of a lithographic process. On the other hand, BAWs need frequency correction, because their frequency position is determined by thicknesses of the layers in the stack, and because the layer thicknesses are controlled by the deposition process, whose accuracy is, however, not sufficient for mobile phones.
In terms of filter performance, SAWs are generally inferior to BAWs and CRFs and therefore cannot be used for most demanding applications. However, SAWs have an intrinsic advantage over BAWs, because they are capable of performing an unbalanced-to-balanced conversion from ingoing to outgoing signals (mode conversion) without effort or extra cost.
On the other hand, BAWs with a single piezoelectric layer exhibit high performance, but they lack the mode conversion option. Therefore, additional effort is needed, for example, via external baluns, and the BAWs thus lose some performance and require additional cost and/or additional space on the board. The CRFs combine the advantages of the BAWs in terms of high performance with the advantages of the SAWs in terms of simple mode conversion. But without frequency trimming, the production of CRFs have a low manufacturing yield, because only a limited quantity meets the required specifications. This is a decisive disadvantage in the manufacturing of CRFs and, in particular, results in increased production costs.